Phenolated and hydroxylated aromatic compounds are one of the main sources of industrial pollution.
Phenolated residual water is found in the effluents of industries involved in the manufacturing of pharmaceutical products, plastic materials, coals, tars and their derivatives, pesticides and dyestuff among others.
The residual phenol concentrations vary widely, depending on the type of industry involved. These concentrations may attain several grams per liter and since it is known that phenols are toxic to living organisms even at very low concentration levels, it has become necessary to develop purification techniques for treating phenolated wastewaters.
However, these techniques have not been very numerous, have almost always involved substantial investments, and, above all, they have not been totally effective. So far, the most effective way to dispose of soluble or suspended organic pollutants in aqueous systems has been to chemically oxidize the aromatic contents either totally to carbon dioxide or partially to acids which are easily degradable by further action or microorganisms.
In the light of the numerous studies performed on the oxidation of phenolated wastewaters, it can be concluded that there are two key aspects which have to be looked upon. They are the chemical steps leading to destruction of the toxic soluble organic material and the configuration of the reactor system in which contacting between liquid and gas phases is made.
The chemical steps leading to oxidation of aromatic compounds are relatively well understood. Basically, oxidation is initiated by the formation of hydroperoxide radicals leading to hydroquinones and quinones and followed by further ring opening and destruction of the aromatic structures.
Since oxidation is undoubtedly the most effective treatment of phenolated wastewaters, many variations of this method have been developed. It is clear although that a flexible and inexpensive purification process has long been sought after, and numerous publications attest these facts.
It has been proposed to effect oxidation treatment by ozone or permanganate. However, these two products are extremely costly and the use of permanganate results in the production of large quantities of undesirable sludge.
Treatment by chlorine has also been considered to be interesting, but it frequently produces toxic chlorophenols and this opposes the achievement of the desired aim, which is precisely to avoid the formation of such undesirable intermediates.
Oxidation using hydrogen peroxide mixed with a salt of ferrous iron as catalyst, conventionally known as the Fenton reagent, has also been proposed and this process was found to be among the most effective ones. However, it presents some disadvantages, namely the necessity of introducing ferrous iron which must be separated after processing, acid pH that is strong enough to attack the reactor walls, very high production costs and finally hydroxylation of the hydrocarbides which may be contained in the wastewater to be purified.
The concomittant use of UV light, temperature and acoustic energy to trigger the free radical oxidation mechanisms has also been reported. Finally, direct wet air oxidation using HSO.sub.5 as a catalyst has been reported and applied to the oxidation of toxic phenolic compounds in wastewaters.
In the use of gaseous oxygen with or without a catalyst, the contacting between the wastewater and the oxygen containing gaseous phase is almost always effected by bubbling the gas through the liquid using a variety of agitation systems. However, it will be understood that mass transfer limitations are encountered in current technologies since the gaseous oxygen has to diffuse through the gas-liquid interface using the inherently low external surface area available in the gas bubbles. Low oxidation rates are thus obtained necessitating long treatment times. This results in massive technologies having significant investing and operating costs.
Thus, in the light of existing technology, it would be highly desirable to provide a new method for treating contaminated wastewaters without leading to undesirable stable reaction intermediates that would be rendered more efficient by improving mass transfer between the contaminated waste and the oxidizing gas.